Now that third week officially ended, I am sure all of you have at least a basic design for your robot and a good idea of how it might look like at competition, and done a lot of testing on how to manipulate the goals and/or soccer balls. Depending on your progress, your team might have full size foam/cardboard prototypes, or even metal prototypes for some of you more experienced teams. Some teams out there will have a metal driving base, and some other teams might still be trying to figure out what gears/sprockets/wheels to buy.

If you are like me, or any engineers on anyone of the teams out there, you would be thinking about motors and gears (or sprockets, depend on your preference)… So this newsletter is made especially for people who like to stay up all night thinking about gear ratio, or what motors would fit where, or how to put together a 4 motor drive train.

Specifically, this edition of newsletter will include ALL you need to know about motors, gears/sprockets, and drive train from this forum.

So, without taking any more of your time, here is the “Motors and drive train edition” of “Fresh from the forum”.

-----------------------------------------
I. FIRST 2002 Motors in General
-----------------------------------------

Team 639 put together a page with picture of all the motors that came with the kit of parts this year. Take a look at the pictures @ http://www.startateam.com/639/motors.php if you aren’t familiar with the motors and their name already.
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=1469

These are the only motors you can use on your robot (and the two servos that came in the kit): 3 Drill motors with transmission, 2 Chiaphua motors, 2 fisher price motors with gearbox, 1 Mabuchi motor (=fisher price motor), 2 globe motor with gearbox, 2 seat motor with attachment, 2 Jideco window motors (left and right), 1 Johnson motor, 1 torque motor. These motors are covered under the “parts found in the Kit of parts” rule, and you are limited by these numbers. Any more of the same kind of motor can only be used as replacements. Info regarding ordering spare motors can be found in the manual (should be after “The robot”). Also, you have to watch out for reusing motors from past years, as there’s no van door motors in the 2002 kit, and window motors and fisher price gearbox are different this year.
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=1588
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=1590
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=2131

Chris Mikus did a wonderful job of putting together all the specs of motors available to use through out these few years (Thanks Chris!). It is up in his webpage, @ http://www.nuts4first.net/ There are a lot of really good guides of different aspect of the FIRST competition, especially useful for rookie teams and beginner teams out there. For the purpose of this newsletter, I will only point readers to a specific part of the web page: the library @
http://www.nuts4first.net/html/library.html with description of what’s in here. If you click any one of the post, it will bring you to an index page of where the documents are. The one with all the specs of motors is: http://www.nuts4first.net/library/Motors.htm
Just make sure you look at the Jideco window motors instead of the old one. Other than that, the chart is really easy to understand. Also, I will put the specs below in the “in depth discussion of specific motors” just so everything is together in one place.
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=1321

About motors in general…
Motors is a good way to power components on your robot. The motors from the kit are electric motors powered by the 12v battery. It will worth your time to learn about how motors work, and characteristic of each of them. A lot of good paper inside the white paper section of chiefdelphi.com, you want to pay attention to is the two motors presentation by Joe J. and Mike C. I also put up a lecture note with detailed description on what I learned from those presentations. They are all at http://www.chiefdelphi.com/forums/papers.php?s= (page 2 & 3)

Also, here is a good thread about physics behind motors and gears…
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=1873
“But, with electric motors, torque is related to current, and speed is related to voltage. And they are locked in a struggle with the internal 'resistance' of the motor.” -Lloyd Burns
“The most important about the electric motors we get from the kit is the speed torque curve… you should really pay attention to it… So basically, the speed torque curve explains how a motor will react under load.” –Ken Leung
“I'm sure most people on this thread are aware by now, but torque/rpm curves of DC electric motors are MUCH DIFFERENT from those of internal combustion engines.” -Kit Gerhart

And a good post by Andy Baker about applying the motors to your robot…
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=1179
“--The globe motors and window lift motors are also good for actuations...
--Seat motors are mainly used these days to collect balls (by driving a spinning roller)
--Torque motors were only used as triggers last year...” -Andy Baker

A little general advice… With the new chiaphua motors in the kit, there are new engineering challenges you have to think of when designing your robot around those motors, especially with this year’s game and all the robots bumping into each other. Just make sure you give enough thoughts before you put motors on robot. ;)
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=1828
“Holy cow, what a lot of trouble two motors can cause! With the addition of the Chiapua motors, we have some new engineering challenges. I don't know about the rest of you, but here are some of the things I've had to to investigate for the first time since college” -Andy Baker
“Isn't it nice to have to THINK about what to do with the various motors in the kit? It has been a no brainer for so long which motors to drive with. Now comes a new challenge. I LOVE having new motors to play with.” -Joe J.

About Heat sink…
If your motors are running hot, try using heat sink or mount fans next to the motor.
“While we were running our robot we noticed the BOSCH and FP were running a little warm, so we found some heatsinks that are BAM extruded aluminum.” - Matt Reiland
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=2141

About the tapped holes on the motor…
“M4 on the drills, M3 on the Fisher Price, M6 on the window, #10-32 on the Chiaphuas..” -Joe J.
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=1833

Once people found out the chiaphua motor is in the kit this year, it became the center of discussion. So I will start this section with the Chiaphua motor.

Chiaphua motors (2) specs- #10-32 for the tapped holes

Joe J. posted a lot of info about the Chiaphua motors, which can be found in this thread.
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=1457
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=1576 (excel spec sheet)

Team update 3 came up with a similar chiaphua specs
http://www.usfirst.org/robotics/doc_art3.htm
with a 5600 RPM free speed, ~3.4Amp free current, ~1.8 N-m stall torque, 112.4 Amp Stall current, which come out to be about 280 Watt Maximum motor power. Not much different than Joe J’s spec.
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=1992

About Chiaphua mating gear...

You will notice that the output shaft of chiaphua motor have some non standard teeth. It is a 36 Diametral Pitch, 9 tooth, enlarged, 25 degree pressure angle gear AND it is enlarged. The kit came with a gear cluster that can mate with the shaft. It’s a 44 tooth with a 16 tooth next to it, and a ¼” hole in the center. You want to use the 44 tooth side to mesh with Chiaphua’s output shaft, and the 16 tooth side has a 32 DP and a 20 degree pressure angle
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=1635
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=1487
The material of the gear cluster is a pretty good material, so you can just put it on a ¼” shaft and it will spin fine. Although, just to be save, you can put mount the ¼” shaft in bearings and lubricate the gear
“The gear has some brass content (copper, etc) and is a pretty good bearing.” -Paul Copioli
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=1826
“The metal of the gear that mates with the Chiaphua gear is
"FX-2008-90HT copper infiltrated steel heat treated to Ra 68"
A friend looked up the spec. It has a Min Ultimate of 90,000psi.” -Joe J.
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=1971

A picture of a quick gearbox for the chiaphua, take a look at how the mating gear go with the chiaphua motor output shaft. It is recommended that you support the gear cluster on both side of the shaft.
“Motor mount screws are 10-32, spaced exactly 2.00" apart
The center "bulge" is .75" in diameter
The centers of each gear (shaft, big gear) is .740" apart.” –Larry Barello
http://64.119.166.94/forums/showthread.php?s=&threadid=1571
As for buying a replacement mating gear for chiaphua… The only suggestion so far is to spend around $240 to buy the whole assembly from Atwood. Any better suggestion?
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=1984

About gearing the chiaphua down…
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=1659

Un-answered questions:
What’s a good way to figure out a best ratio for chiaphua for a fast drive train?
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=1764
Is it safe to press gears onto chiaphua pinion?
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=2071

You can only the drill motors that came in the kit of parts… That means a maximum of three drill motors on the robot. Any more drill motors have to be used as replacement parts.

These drill motors have been in the competition for a long time, so most teams shouldn’t have a problem applying them on the robot. Just remember to mount it properly, and keep side loads away from the drill motor output shaft.
Tons on great advices on drill motors in the Nuts4FIRST library @
http://www.nuts4first.net/html/library.html
And there’s the two basic MUST KNOW in Chiefdelphi.com white paper section:
@ http://www.chiefdelphi.com/forums/papers.php?s=
Gear shifting and motor mount by Andy Baker,
Dr. Joe’s drill advice

About drill shaft coupling…

You can buy these drill motor shaft couplings from Small parts to attach a 3/8” shaft to drill’s threaded output shaft. They are pretty reliable, as long as you use them right. Just remember to drill the hole out so the 1/8” dowel can fit (at least some sort of press fitting so the pin don’t fall out) in and lock the coupling in place.
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=1885

About speed difference between forward and backward...

People observed an interesting characteristic when using drill motor for the drive train- the drill go faster when spinning forward. It is most noticeable when you mount the two drills outward away from each other (tank drive), causing the motors to spin at different direction. When the robot moves forward, the speed difference cause the robot to turn slightly toward the backward spinning motor. Bottom line is, if you drive train is really fast, you will probably notice the difference… If not, then the robot should go pretty straight. If you can’t mount the motors the same direction, then it is recommended that driver compensate for this instead of limiting voltage to the faster motor. Also, you can use the gyro (Yaw Rate sensors) to sense changes in drive train direction and program the robot to drive straight.
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=1600
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=1750

About O-ring for drills

If you are not planning to shift gears with drill transmission, and willing to take apart the drill transmission, then you can manufacture these O-ring (or buy pipes from small parts and cut a ring out of them) to fit inside the gearbox to lock it in either high gear or low gear. It is a really reliable way to keep transmission in gear.
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=1841
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=2027

About buying spare parts…

You should look up a Skil-Bosch service center in your area to buy spare drill motors. The part number for drill motors is 3310k-10.
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=2045
Bosch have a homepage @ http://www.boschtools.com/homepage.htm
You should be able to look up service center near you, or where to buy the drills from them.

Turns out Fisher price motors and the Mabuchi motor are identical.
“I will only say that I am 99% sure that the motors are the same.” -Joe J.
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=1560

Teams found out that the fisher price gear box isn’t really reliable, so they did various modifications to fix it up.
“Our team found that the problem with the Fisher-Price gearbox is not so much that the gears are bad, but that one of them is seated poorly… Once we drilled right through the gearbox to secure the shaft more solidly, the gearbox worked much better.” - ianmackenzie
“The last gear was breaking teeth, and, amazingly, it was on the gear and not the pinion… made an aluminum ring gear and bolted it onto a turned down FP gear.” -Bill Beatty

Also, after a close look in the fisher price gear box this year, someone noticed that the gear box is different, and that the fisher price motor actually spins at 20,000 RPM… So, you should watch out for using fisher price motors from last year as replacement…
“and has a 21T gear on it as opposed to the 16 tooth- the net effect of the higher speed and the larger drive pinion is the higher output speed. The last 2 stages of gearing have been changed as well-“ -EZRrider
“Is it only me who has noticed the final stage of the Chipua drive 'copper' gear will fit perfectly into the FP gear box?” -Dr. Bot
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=1630
“after running both sets against each other i have foudn the following
1: there seems to be more torque produced in this years set
2: this years set seems to be more sturdy takes more abuse” -Andrew Dahl
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=2131

Another fine addition to the kit this year… With maximum motor power of 40 Watts, this little guy is even more powerful than a seat motor or a window motor.
http://64.119.166.94/forums/showthread.php?s=&threadid=1586

The above specs came from Joe J’s excel sheet of motors specs. There’s a different number on FIRST’s manual for the motor without transmission, it is very possible that the one in manual is a typo. But it shouldn’t matter too much, since most teams use the globe motors with transmission on.
“With the "drive assembly" the no load speed is 87 RPM and the stall torque is 150 In-lb (16.95 N-m). The no load speed without the "drive assembly" is just slightly faster (97 RPM) and the stall torque is much much lower (30 oz-in, or 0.212 N-m).” -patrickrd
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=1490

About dimension for the globe motors…

I don’t remember anyone mentioning about dimensions for the globe motors online, and there aren’t any blueprints from FIRST. So, it is very likely that you will have to grab a pair of caliper and measure the globe motor itself for dimension.
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=1731

Which come out to be about 14 Watt Maximum motor power (@ 12V).
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=1799
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=1821
There are no track/tape drive for window motors this year… Mainly because it is actually a different window motor. So, you may not reuse the window motors from last year.
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=1921
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=1987

Belimo torque motor (1) specs

You can find specs for the torque motor at this link:
http://www.belimo.com/mft/ActuatorDetail.cfm?Product_ID=4764
http://www.belimo.com/mft/ActuatorSeriesMain.cfm?Series=LM&Spring=0

About wiring the torque motor…
“Cut a PWM cable in two pieces, then use the female end to plug to the torque motor. I believe you plug it into pins 1, 2 and 3, using two of them as positive and negative. There is a drawing of the polarity right above those pins, follow it. :)” - Manoel
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=1991

FIRST opened up the rules regarding gears and sprocket this year… You can get whatever gear you want, at any amount as long as you robot fit inside the dimension and weight limit. Of course, these have to be off the shelf products, meaning, they should be commercial products out there open to everyone to purchase. There have been a debate about whether custom making your own gear/sprocket is bounded by the additional hardware list and small parts catalog rule (if you can only custom make gear/sprocket with allowed material, or if you can use whatever material you want under the any gear/sprocket you want rule). But without a clarification from FIRST, your best bet is still to buy gear/sprocket from stores or online stores. Try to stay within rules and its spirit… Gracious Professionalism is KEY.

Before you decide to buy the gear/sprocket, do some calculations for what kind of gear/sprocket you would need for your component. After all, you want your gears strong enough to withstand the load, AND save weight and cost at the same time. For example, Delrin gears are probably not too good for the drive train, but steel would. Also, you might want to buy different tooth gears to try out different ratio. There are only so many calculation you can do before slapping on different gears and try out how they will work out.
“It's possible you might get by with a delrin gear early in the drive train (say the 1st gear that mates with the pinion) but this depends on the ratio. My feeling is you would certainly experience a disaster at some point early in the competition, why risk it.” -Ed Sparks
“Brass is a very good gear material. The yield stress of many brasses is higher than that of many steels. Also the wear & friction properties are often better than steels.” -Joe J.
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=1957
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=1812

If you want to learn about calculation of gear’s strength, check out this thread with a good approximation that’s sufficient for the usage on FIRST robot:
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=1873

About sources for gears:
There are a lot of online places you where can buy gears/sprockets… Here are some suggestions from different people:
“A few good small components sources:
www.wmberg.com
www.allieddevices.com” - Keithicus
“mcmaster.com and stock drive products (sdp-si.com) are good sources. Also, motion industries is a good source.” -Joe J.
“http://www.globalspec.com
You can search across the industry for the part you want. It is particularly useful for gears and sprockets.” -ChrisH
“We found a company called Motion Industries that carried the identical part. Turns out they have about 500 outlets around the country (check their locator at http://www.motionindustries.com/ ).” -dlavery
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=1975
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=1882

About Buying 0.7 modulus gears…
“Go to Pic-Design.com
They are the only source I am aware of that sells metal 0.7 Module gears.” -Joe J.
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=1890

About pairing up Chiaphua and Drill with a gearbox...

A little discussion about making gear box for pairing up bosch and chiaphua
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=1949

Which lead to another topic, turns out not a lot of people understand how gears and ratio work. It might seem confusing at first, but actually quite easy to understand. I included a little section about understand gears and ratio in my WRRF motor selection lecture notes, which can be downloaded @ http://www.chiefdelphi.com/forums/papers.php?s=
But basically, gears and sprockets can be use to reduce speed and increase torque, or the other way if necessary. Also, they be used to change direction of rotation or motion, and transfer power from one place to another. Basically, when gears are being driven, there is a certain torque and teeth force on that gear. And if you put different size of gears together, the gears pushing each other will generate different torque on each gear. Just think about this in terms of lever arm and torque. If you understand the concept of torque, then you should understand this.

About CVT…
A really interesting about Continuous Variable Transmission… This kept me awake for a couple of nights. ;) The discussion starts some where toward the bottom.
“Picture this ... the output from the drill motor transmission is the sun gear for another planetary stage. We use the fisher price motor to rotate the ring gear” - Paul Copioli
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=1179

Ken Leung

01-28-2002, 09:49 PM

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IV. Drive train
-----------------------

Drive train is the most important component on you robot, especially with this years game where speed and pushing force is really important. After all, what good would a robot do if it can’t move around and do what it’s designed for.

A few of us were trying to understand how a drive train work, and we came to a few conclusion:

A drive train will go as fast as its drive motors spinning at free speed after reduced by the gear ratio, and slow down by internal friction. (Just the robot running by itself on the field)
And the drive train will push as strong as the drive motors pushing with stall torque after multiply by the gear ratio, OR with a torque just before the motor start tripping the circuit breaker, OR pushing as strong as possible before wheels start slipping.
Check out this thread for more detailed discussion
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=1114

About Drive train advice…
Check out this thread:
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=1179

calculation for speed...

It’s not easy to figure out how fast your robot goes just by calculation, with all the internal friction in drive train, and different combination of motors. You best bet is still put the drive train together, run it with enough weight of your robot, and time the robot. You will be surprised how easy it is to put on different gears and try out different speed/torque combination.
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=1907

Once you tested the robot for speed, it is quite easy to figure out how much overall internal friction is in your system.
“You can see by the figures for free speed and the geared down speed (multiplied back up to the motor speed input to the gears) how much "friction" the gear box represents. Then, with the a Speed-Torque curve for the motor in question in front of you, enter on the torque axis, go up to the line, the at the line, move horizontally to the left, and read the speed. ” -Lloyd Burns
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=2025

Gear shifting

With the different demand on the drive train this year (speed to get to goals FAST, and torque to push/pull goal(s) with other robot(s)), a lot of people are considering gear shifting as the solution. By shifting to between two gear ratio, your drive train can run either with high speed/low torque, or high torque/low speed. A lot of teams take advantage of the gear shifter on drill transmission, but there are a lot of other ways as well.
“Last year we used drill motors for propulsion. We used servos to shift gears. The problem is you had to be standing still to shift.” - Wayne Doenges
“See if someone has a picture of the HOT BOT from last year they had a trick shifting mechanism using little air cylinders. I don't know if they used it but it looked really cool” -Matt Reiland
“Our team is going to try to shift on the fly by having 2 sets of drive wheels that are always spinning (one set for high gear, one set for low gear) and putting the high gear wheels down with a pneumatic.” - thedillybar
“A transmissiom would be cool - anyome thought of using something smple like a bicycle chain dereiler” -Dr. Bot
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=1845
67’s shifter, and 131’s wheel switcher
http://www.chiefdelphi.com/forums/showthread.php?s=&postid=9845#post9845
Team 45 Technokat’s servo gear shifter, one of the best design for using servo to switch gears. However, I do recommend you spending time developing your own design… It is a really good learning experience that way (taking apart drill transmission to understand the gear shifter... and figuring out a good mechanical design to apply the servo).
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=1800

Multiple motors on drive train

Another solution to needing speed and torque on the drive train is adding more motors. More mechanical power means additional speed AND torque, although it is hard to get two type of motors working together (not impossible). The idea is that you have to match up the motors such that they are going at the same speed, and have similar reaction under load. You should figure out exactly what you want your robot do, before deciding if you want to gear shift, or use multiple motors…
“The bottom line is that the speeds are required to be in some ratio that is fixed by the gearing. Assuming the same voltage to the motors, the motors then simply act like the SUM of the two motors. The speeds are equal (or geared to a ratio) the torque out is like the sum of each motor at that speed.” -Joe J.
“Last year we connected the drill and Fisher-Price motors end to end, ran the output of both through the drill gearbox, and drove our track drive from the output of the gearbox… We had them running at the same speed and it worked pretty well.” -Kit Gerhart

One concern people have is that more motors might draw too much current. However, turns out the 60 amp circuit breaker is more tolerant that it says. Both from people’s experience, and from team update 2:
“According to a post on another thread, at a load of 100 A, the breaker takes nearly two minutes to trip and takes 8 seconds at 200 A.” -Kit Gerhart
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=1623
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=1175
“Also, you should design your drive system so that it is difficult/impossible to actually stall your drive motors” -Joe J.
http://64.119.166.94/forums/showthread.php?s=&threadid=1571

About using chiaphua’s as drive motor…
“If you look at the specs, you will see a top speed of 5500 RPM. I'd say that would take more than a 3:1 or 4:1 reduction to acheive a drivable speed.” - Jeff Waegelin
http://64.119.166.94/forums/showthread.php?s=&threadid=1728

About pairing up Drills and chiaphua’s…
Want to use both drills and chiaphuas in the drive train? Take a look at this thread…
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=1544
And this:
“I suppose that you might get away with gearing up the Chiaphua’s and stuffing both into the Drill Trans. One problem is going to be inertia. You may be able to get away with the static loads, but the rotary inertia of the two motor (and gearing) may cause a very large dynamic loading condition.” -Joe J.
“Last year's game gave us a false sense of security when it comes to linking motors together.” -Paul Copioli
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=1693

Care to share how much motors you are using on the drive train? Cast a vote in this poll…
What motors for your drive train? Poll
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=1644

Well, what can I say… With a drive train on your robot, it is probably a good idea to use wheels or treads on that drive train. ;) This year’s rule say that you can use skyway wheels, any amount/any size… And any length of chain/belt, and the timing belt ruling changed into “Belt - Any size, Any length for use with pulleys”. So, go crazy, put on however many wheels/threads you see fit. Just remember, you don’t necessary get more traction by using more wheels/treads, just that it’s easier on the carpet if you do use more. ;)
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=1724
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=1808

You can also make your own wheel, as long as you are using legal material.
“We made our own wheels. We used 6" circular solid aluminum (available from SPI) and took out some weight with holes, and added a built-in hub and bearing.” -Jeff Waegelin
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=2054
Speaking of using legal material, a lot of times people have left over stocks of material that’s listed on small parts catalog, and want to save money by using those stocks instead of ordering fresh new ones from small parts. Now, the rules aren’t really clear about this, so try to stay within the rules, and the spirit of the rules… Gracious Professionalism is KEY.
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=1965

Traction
Traction is what make your drive train work. If your wheels have no traction at all, your robot will be sitting at the same place burning carpet. Traction is also a limiting factor of how strong your robot can push. It is very likely that your wheels start slipping before tripping circuit breakers or stalling motors. A lot of teams learn the lesson about traction the hard way from last year’s game… They used unmodified skyway wheelchair wheels, and that wasn’t enough traction to get their robot up the bridge. On the other hand, you don’t want too much traction, which will lead to stalling motors or damaging carpet… So you have to give it a lot of thoughts and experiment before you get it right.
“You really don't want unlimited traction, because it will lead to stalled motors & or damaged carpet. Neither of these are good things. If you don't worry about traction in your design, you will not be likely to realize the benefit of all that torque, but if your traction is too good, you'll find yourself tripping breakers and shredding carpet.” P.J. Baker
“Get a fish scale. Hook it to your robot parallel to the plane the center of your wheels are on. Increase power to motors until you loose traction… Why the scale? The force you should read should be equal to your coefficent of friction times your normal force (weight). F=uM.” -Jon Lawton
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=1979

Another good way to measure friction
“It is easy to make a test rig to measure the coefficent of friction. glue a piece of the sample carpet to a board and cut a couple on equal sized squares of wood, and glue various samples of possible tread material on them. add some weight, a spring scale and measure away.” -Dr. Bot
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=1969

Wheels
Some of you should rethink about using those wheelchair wheels that came with the kit as driving wheels. They weren’t designed to push 130 lbs robots across a carpeted field. Modify them for more traction, or use different wheels.
“Buying wheels, if they make high traction wheels great, but those wheel chair wheels are useless. That is, unless you cut them up.” -401 Mentor
“Last year our team used the wheelchair wheels. We used a sloldering iron with a "V" shaped tip to cut tread into the tires. We had excellent grip.” - Wayne Doenges
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=2015
A lot of cool wheel advices toward the end this thread @
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=1424

Wheels vs. treads…

There is a good debate between using wheels and using threads. Basically there are pros and cons for each side. Wheels: while they are really efficient and easy to apply, they don’t provide nearly as much traction and contact surface. Treads: while they provide a huge amount of traction, they are hard to maintain and develop. Take a look at this thread for that discussion… There also great advices/ideas for wheels and treads in that discussion.
“have you ever seen the Technokats (45) treads? they had the most efficient and fastest drive systems out there. those things could plow over anything they wanted to.” -David Kelly
“In general, I would say that treaded systems are more difficult to debug and maintain. #45 has made it work wonderfully for them, but I would advise that your 1st robot have a simple 4WD, tank steering setup. It will be easier to build and you'll have a lot more time to debug it.” -P.J. Baker
“The TechnoKats have awesome drives. Their method of doing tracks is very efficient --- FOR A TRACK. I disagree with any claims that they are as efficient as wheels.” -Joe J.
“For us we wanted alot of grip of course. So what we did was
wrap timing belts around the wheels wit the teeth side out and we had superior traction.” - Andrew Rudolph
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=1424
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=2109

Treads
As I said, treads are difficult to develop and maintain… A lot of teams have good and bad experience with using treads on the drive train. Take a look at the advices they have about setting up a tread system on your drive train @
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=1805

About wheel hubs…

Small parts sell pre-made wheel hubs for the skyway wheelchair wheels that came with the kits. For those of you who are using those wheels, the wheel hub is a great solution to attaching sprockets onto the wheel. They are really reliable, and teams haven’t complain anything major about those hubs. Meanwhile, those of you who aren’t using wheelchair wheels, you will have to figure out a way to attach sprocket/gear next to the wheels. Shouldn’t be too hard since a lot of the other wheels from skyway have bolts in them already.
“If you look in your Zone Zeal rulebook you will find the hubs. They are located on the pages after the kit and additional parts sections. One bit of advice. When tightening the hubs onto the wheels, be careful not to over tighten some of the bolts. You WILL distort the wheels causing them to wobble.” -Wayne Doenges
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=1777
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=1950

Also, skyway seems to sell keyed wheel hubs. This should be useful for those of you who are driving wheels with a driven shaft, such as having a direction connection between motor output shaft and wheel. Although, another solution is to attach gears/sprockets next to the wheel, and use chain or gearbox to drive that wheel.
“Skyway (from the fax) looks like they make others hubs that have keyways. I don't know if you can swap hubs but you might be able to purchase other hub/wheels that would attach by design.” -iscrc2
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=1948

About wonder wheels

Wonder wheels, also known as holonomic wheels or omni-wheels, are side slip wheels with very little side friction. The way they work is that people manufacture these wheels out of legal material, and insert little free spinning disks or rollers around the edge of the wheel. Resulting a wheel that push forward with the gap between each disks/rollers digging into the carpet, and side slip on the free spinning rollers/disks around the wheel. With wonder wheels as two of the wheels on your tank-drive, the drive train can spin and turn around smoother, because the drive train will be spinning around the center of the two normal wheels instead of the center of all four wheels. One disadvantage is that the wonder wheels side of your drive train can easily be pushed from the side by another robot.
“The setup we used last year provided a good deal of traction left/right IIRC as we didn't have all 4 wheel mounts as omni-wheels. Instead we had a 4 wheel drive differential system with 4 omni-wheels in front, and 2 wide wheels in the rear with really good traction. This let us have 4-wheel drive and a good turning radius.” -Patrik
“We thought about using the side-slip wheels this year but decided not to. The reason, there is no way to stop your robot from being pushed around from the side.” -Wayne Doenges
http://www.chiefdelphi.com/forums/showthread.php?s=&threadid=1551